From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
Recently, superconductivity was discovered in a new class of materials. See, for instance B. Batlogg, Physica 126,275 (1984), which review superconductivity in barium bismuth lead oxide, and J. G. Bednorz and K. A. Muller Zeitschr f. Physik B--Condensed Matter, Vol. 64, 189 (1986), which reports superconductivity in lanthanium barium copper oxide. The latter disclosed material had an onset temperature in the 30 K. range.
Especially the latter report stimulated worldwide research activity, which very quickly resulted in further significant progress. The progress has resulted, inter alia, to date in the discovery that compositions in the Y-Ba-Cu-O system can have superconductive transition temperatures above 77 K., the boiling temperature of liquid N.sub.2 (Phys. Rev. Letters, Vol. 58, Mar. 2, 1987, p. 908; and ibid, p. 911). Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of compositions and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c above 90 K. (see the U.S. patent application Ser. No. 024,046 entitled "Devices and Systems Based on Novel Superconducting Material," filed by B. J. Batlogg, R. J. Cava and R. B. van Dover on Mar. 9, 1987, co-assigned with this and incorporated herein by reference, which is a continuation-in-part of an application filed by the same applicants on Mar. 3, 1987, which in turn is a continuation-in-part of application Ser. No. 001,682, filed by the same applicants on Jan. 9, 1987).
As discussed in detail in the above referred to U.S. patent application, prior to the discovery, at AT&T Bell Laboratories, of the nature of the superconducting phase and of compositions and processing conditions that can result in substantially single phase oxide superconducting material, the expressed belief in the field was that the Y-Ba-Cu-O superconducting material is multiphase material, with only a relatively small part (variously considered to be about 24% or about 2%) being superconducting. Significantly, it was also speculated that the high temperature superconductivity of the material may be associated with interfacial manifestations, and may not be identified with perovskite or tetragonal layered structures.
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. For obvious reasons liquid hydrogen and neon would not be very desirable refrigerants, and therefore it is liquid nitrogen (B. P. 77 K.) that is considered to be the lowest boiling point convenient and inexpensive cryogenic refrigerant. Attainment of superconductivity at liquid nitrogen temperature was thus a long-sought goal which for a long time appeared almost unreachable.
Although the "holy grail" of superconductivity, T.sub.c &gt;77 K. has now been attained, there still exists at least one barrier that has to be overcome before the new oxidic high T.sub.c superconductive materials can be utilized in many technological applications. In particular, techniques for forming superconductive bodies of technologically significant shape have to be developed.
To date known oxidic superconductive bodies are substantially three-dimensional bodies (e.g., pellets, disks, tori), i.e., all three dimensions are of substantially the same order of magnitude. Although such three-dimensional bodies may have specialized utility widespread use of the new high T.sub.c materials will occur only if superconductive thin films, and sheet-like and filamentary superconductive bodies can be produced from the material.
According to a so-far unconfirmed report in the press, one laboratory has succeeded in producing a superconductive thin film from one of the oxidic high T.sub.c materials by an evaporation technique. Evaporation as well as other thin film deposition techniques generally can be used to deposit layers of limited thickness (typically &lt;5 .mu.m) onto a substrate.
No technique for producing superconductive bodies having a small dimension greater than is obtainable by thin film techniques but less than is found in the prior art three-dimensional bodies has yet been reported. Such bodies, which can be free-standing or can be in contact with a substrate, typically have a minimum dimension in the range from 5 or 10 .mu.m to about 1 mm, and have at least one dimension which is much greater than the minimum dimension. If a body has two dimensions that are approximately equal and in the above range then we will refer to such a body as "filamentary". An example of a filamentary body is a thin rod. If a body has, in addition to the one small dimension, two large dimensions of approximately equal magnitude, or one large dimension and one intermediate dimension then we will refer to the body as "sheet-like". Examples of sheet-like bodies are sheets, wide strips, shaped portions of a sheet, and strips or lines in which both the thickness and the width are within the above range.
If filamentary and/or sheet-like superconducting bodies could be produced from the new high T.sub.c oxidic materials it is certain that they would find significant technological application. It will be recalled that prior art superconductors are used, in addition to thin film form, essentially only in filamentary form (as wire), and in strip-form. Furthermore, because of the novel properties of the oxidic superconductors, it is likely that filamentary and sheet-like superconductive bodies would find uses in ways that were not possible or practical with the prior art metallic superconductors. Thought is given here, for instance, to assemblies of sheet-like superconductive elements.
For one overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and Machines, Plenum Press 1977; S. Foner and B. B. Schwartz, editors, Superconducting Machines and Devices, Plenum Press 1974; and S. Foner and B. B. Schwartz, Superconductor Material Science, Metallurgy, Fabrication, and Applications, Plenum Press 1981. Among the applications are detection and measurement apparatus based on, e.g., the Josephson effect or electron tunneling, power transmission lines, rotating machinery, and superconductive magnets for, e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage. The prior art has considered these actual and potential applications in terms of the prior art (non-oxidic) superconductors. It is expected that the above and other applications of superconductivity would materially benefit if high T.sub.c superconductors could be used instead of the previously considered relatively low T.sub.c superconductors.
In the previously referred to co-assigned U.S. patent application Ser. No. 024,046 it is disclosed that the "calcined oxidic powder is "formed into a ceramic body of desired shape by standard ceramic processing techniques such as hot or cold pressing, extension, slip casting, or other such technique appropriate to the geometry of the desired (green body) object."
Prior art ceramic techniques can indeed successfully produce three-dimensional superconducting bodies. However, workers in this field have so far failed to produce superconducting filamentary or sheet-like bodies by any technique, including ceramic techniques.
A known technique, screen printing, has been used in the past to form sheet-like bodies on ceramic substrates. See, for instance, B. Schwartz, Ceramic Bulletin Vol. 63(4), p. 577(1984). The bodies included conductors (including conductors that comprise the conducting oxide RuO.sub.2), insulators and dielectrics. The technique was not used to form any superconductive bodies.
In view of the immense potential importance of high T.sub.c superconductive bodies of technologically useful (i.e., having at least one small dimension and at least one relatively large dimension) shape, a technique for producing such bodies would be of great significance. This application discloses such a technique.